A human intervertebral disc has several important functions, including functioning as a spacer, a shock absorber, and a motion unit. In particular, the disc maintains the separation distance between adjacent boney vertebral bodies. The separation distance allows motion to occur, with the cumulative effect of each spinal segment yielding the total range of motion of the spine in several directions. Proper spacing is important because it allows the intervertebral foramen to maintain its height, which allows the segmental nerve roots room to exit each spinal level without compression. Further, the disc allows the spine to compress and rebound when the spine is axially loaded during such activities as jumping and running. Importantly, it also resists the downward pull of gravity on the head and trunk during prolonged sitting and standing. Furthermore, the disc allows the spinal segment to flex, rotate, and bend to the side, all at the same time during a particular activity. This would be impossible if each spinal segment were locked into a single axis of motion.
An unhealthy disc may result in pain. One way a disc may become unhealthy is when the inner nucleus dehydrates. This results in a narrowing of the disc space and a bulging of the annular ligaments. With progressive nuclear dehydration, the annular fibers can crack and tear. Further, loss of normal soft tissue tension may allow for a partial dislocation of the joint, leading to bone spurs, foraminal narrowing, mechanical instability, and pain.
Lumbar disc disease can cause pain and other symptoms in at least two ways. First, if the annular fibers stretch or rupture, the nuclear material may bulge or herniate and compress neural tissues resulting in leg pain and weakness. This condition is often referred to as a pinched nerve, slipped disc, or herniated disc. This condition typically will cause sciatica or radiating leg pain, as a result of mechanical and/or chemical irritation against the nerve root. Although the overwhelming majority of patients with a herniated disc and sciatica heal without surgery, if surgery is indicated it is generally a decompressive removal of the portion of herniated disc material, such as a discectomy or microdiscectomy.
Second, mechanical dysfunction can cause disc degeneration and pain (e.g. degenerative disc disease). For example, the disc may be damaged as the result of some trauma that overloads the capacity of the disc to withstand increased forces passing through it, and inner or outer portions of the annular fibers may tear. These torn fibers may be the focus for inflammatory response when they are subjected to increased stress, and may cause pain directly, or through the compensatory protective spasm of the deep paraspinal muscles.
Traditionally, spinal fusion surgery has been the treatment of choice for individuals who have not found pain relief for chronic back pain through conservative treatment (such as physical therapy, medication, manual manipulation, etc), and have remained disabled from their occupation, from their activities of daily living, or simply from enjoying a relatively pain-free day-to-day existence. There have been significant advances in spinal fusion devices and surgical techniques. However, the procedures generally include shaping two adjacent boney vertebral endplates to conform to the endplates of the fusion device. The removal of bone from the endplates weakens the vertebral bodies and can lead to device stress shielding, bone remodeling, device subsidence, and device expulsion. Further, known endplates can lead to uneven distribution of loads across the vertebral bodies.
Known artificial discs offers several theoretical benefits over spinal fusion for chronic back pain, including pain reduction and a potential to avoid premature degeneration at adjacent levels of the spine by maintaining normal spinal motion. However, like spinal fusion surgery, the removal of bone from the vertebral endplates typically is necessary, thereby, weakening the vertebral bodies. Further, known endplates cause uneven distribution of loads across the vertebral bodies.
Therefore, a need exists for an intervertebral implant and a method replacing an artificial disc that overcomes or minimizes the above-referenced problems.